Cardiovascular disease is the leading cause of death in the United States and claims the lives of more than 600,000 Americans each year. According to the World Health Organization, cardiovascular disease is the leading cause of death worldwide and claims the lives of approximately 7 million people per year. Further, according to the American Heart Association (AHA), more than five million Americans are diagnosed with heart valve disease, which is a form of cardiovascular disease, each year, and diseases of the aortic and mitral valves are the most prevalent. Combined, aortic and mitral valve diseases affect more than five percent of the U.S. population. Hence, it is clear that cardiovascular disease, and heart valve disease in particular, is a major health concern and impacts the lives of numerous people.
Aortic stenosis (AS), which is a form of aortic valve disease, is a ubiquitous and potentially life-threatening disease that impacts approximately 1.5 million people in the United States and is the third most common cardiovascular disorder in the western world. Aortic stenosis is a general term that characterizes the abnormal operation of the heart valve that separates the left ventricle from the ascending aorta, and AS may or may not be symptomatic. A stenosed aortic valve (AV) that does not open completely leads to abnormal blood flow through the valve and the aortic root. These abnormal flow patterns may lead to increased vascular resistance and insufficient downstream perfusion. In addition, an AV that does not close properly may lead to aortic regurgitation (AR), in which reverse flow traverses the AV during diastole when the valve is supposed to be closed completely.
Mitral regurgitation (MR), which is a form of mitral valve disease, is also a widespread and potentially life-threatening disease. In the United States, the occurrence of MR increases with age. In a study conducted in 2000, at least moderate MR was observed in 0.5% of participants aged 18 to 44 years and in 9.3% of participants aged 75 years or greater. In Europe, MR is the second most frequent valvular disease requiring surgery. Similar to aortic regurgitation, mitral regurgitation is a general term that characterizes the abnormal operation of the mitral valve, which is the valve that separates the left atrium from the left ventricle. When the mitral valve does not close properly, blood may leak from the ventricle into the atrium during contraction of the left ventricle and thereby decrease the pumping efficiency of the heart. In contrast to dysfunctional aortic valves, dysfunctional mitral valves may be repaired and may not require replacement.
The prognosis of patients with severe, untreated valvular heart disease is poor. In the case of AS, for example, clinical studies of untreated patients have demonstrated that survival rates are as low as 50% at two years and 20% at five years after the onset of symptoms. Further, acute mitral regurgitation is poorly tolerated and carries a poor prognosis in the absence of treatment. Therefore, it is evident that patients with symptomatic, severely diseased heart valves should seek treatment.
Accurate clinical diagnosis is instrumental in determining the severity and nature of heart valve disease. The American College of Cardiology (ACC) and the AHA have published medical guidelines that help characterize the clinical indications for valvular heart disease and the corresponding clinical treatments. In the context of AS, diagnosis is dependent on the quantitative values of various blood flow parameters as well as a visual inspection of the valve and its operation. The outcome of a patient examination may be a diagnosis of mild, moderate, severe or critical AS. Per society guidelines, only patients with symptomatic, severe or critical AS may be candidates for aortic valve replacement (AVR), which usually involves open heart surgery. Similarly, the ACC and AHA have published guidelines to help diagnose and treat diseases of the other three heart valves, and these diagnostic methods are based on analysis of medical images and characteristics of the blood flow.
Despite the apparent need for treatment, an increasing number of patients with symptomatic, severe AS are ineligible for open heart surgery and surgical AVR. Ineligibility for open chest surgery may be due to significant co-morbidities, such as high surgical risk, advanced age, history of heart disease or frailty. These patients have a poor prognosis and may benefit greatly from alternative therapies and treatments that do not require open chest surgery.
For patients deemed inoperable or who do not wish to undergo an invasive surgical operation, minimally invasive or transcatheter valve implantation may be an option for improving valvular function, alleviating symptoms, and improving quality of life. Transcatheter aortic valve replacement (TAVR), for example, is a minimally-invasive approach to replace the malfunctioning native aortic valve with a functional prosthetic valve. During a TAVR procedure, a prosthetic aortic valve is typically inserted via a catheter that is introduced via a femoral or transapical pathway. In contrast to surgical AVR, TAVR does not require a sternotomy (incision in the center of the chest that separates the chestbone to allow access to the heart), and a heart-lung machine is not needed because the heart is not stopped. Further, because the TAVR procedure is less invasive than surgical AVR, patients generally spend less time in the hospital, experience shorter recovery times, and may be less reluctant to undergo the procedure. Transcatheter valve implantation may also be an option to repair other heart valves such as the mitral or pulmonary valve. Alternatively, sutureless heart valves provide a minimally invasive mechanism for heart valve replacement.
Despite the apparent benefits of transcatheter valve replacement, there are serious clinical risks associated with the procedure. In the case of TAVR, for example, clinically significant post-procedural AR is a frequent problem and occurs in up to 50% of patients. Further, results from clinical trials suggest a linear relationship between the severity of post-procedural AR and 1- and 2-year mortality, and even mild AR may be associated with increased mortality. Therefore, to maximize the potential benefits of TAVR and minimize the long-term risks to patient well-being, AR should be minimized as much as possible. Other risks of transcatheter valve replacement, which are applicable to all percutaneously deployed heart valves, include stroke, vascular complications, improper deployment, obstruction of secondary vessels (e.g., coronary ostium), and valve migration.
Minimizing the risks of negative complications following transcatheter valve implantation requires careful pre-surgical planning and execution of the procedure. Valvular regurgitation in the presence of transcatheter aortic heart valves, for example, is often due to a large mean annulus size, valvular calcification, and/or improper sizing of the valve. Specifically, paravalvular regurgitation (i.e., undesired, reverse flow—or leakage—that occurs between the perimeter of the prosthetic valve and the aortic annulus) is a frequent occurrence with aortic valves and is often caused by improper valve sizing. In contrast to surgical valve replacement, wherein the surgeon may visually inspect the anatomic structure of the native valve and surrounding vasculature before implanting the prosthetic valve, transcatheter approaches currently rely on clinical imaging techniques (e.g., echocardiography, computed tomography, magnetic resonance imaging) for sizing, positioning, and deploying the prosthesis. These images may not provide accurate anatomic information suitable for precise planning and deployment of transcatheter valves, which may contribute to the relatively high incidence of complications (e.g., valvular regurgitation).
Two-dimensional images of inherently three-dimensional anatomy may provide inaccurate information for planning and executing transcatheter and minimally invasive procedures. In addition, imaging modalities with relatively low spatial resolution (e.g., ultrasound) may be unable to resolve anatomic structures that are critical for pre-surgical planning. In the context of TAVR, for example, relatively low resolution two-dimensional echocardiographic images are known to underestimate the size of the aortic annulus; the size of the annulus is used to select the size of the prosthetic valve. This underestimation of vascular dimension may lead to deployment of a relatively small prosthetic valve and thereby contribute to a high incidence of paravalvular regurgitation because the prosthetic valve is too small to fill the native annulus. In contrast, relatively high resolution two-dimensional computed tomography (CT) imaging is known to overestimate the size of the aortic annulus, and prosthetic valve sizing based on CT measurements often leads to a lower incidence of regurgitation. Hence, while proper sizing and pre-procedural planning of transcatheter and minimally invasive heart valve procedures is widely recognized as an essential component for maximizing clinical benefits, the means by which these heart valves are sized requires appreciable clinical judgment and is prone to error.
Therefore, it would be very desirable to have a system and method for accurately assessing the anatomic size and morphology of heart valves and the surrounding vasculature. Such a system and method would ideally facilitate proper selection, sizing, positioning, and pre-surgical planning of prosthetic heart valve procedures. Such systems should not expose patients to excessive risks.